Two exemplary and very similar inspection systems for pattern defects in photomasks employed in the large-scale manufacture of semiconductor devices and integrated circuits are described in U.S. Pat. Nos. 4,000,949 of Watkins and 3,614,232 of Mathisen. The systems of Watkins and Mathisen contemplate the simultaneous inspection of all of the dies on a photomask which contains a regular array of normally identical dies to detect the presence of nonperiodic defects, i.e., defects in one die not identically repeated in the remaining dies of the array.
This task is accomplished by illuminating simultaneously all of the dies of a specimen photomask with collimated coherent light emanating from a laser to develop a composite diffraction pattern whose spatial distribution is the combination of two components. The first component is the interference pattern of the array of dies, and the second component is the interference pattern of a single die of the array. The first and second components are sometimes called an inter-die interference pattern and an intra-die interference pattern, respectively. The light transmitted by the photomask strikes a double-convex lens which distributes the light on a spatial filter positioned a distance equal to one focal length behind the lens.
The spatial filter comprises a two-dimensional Fourier transform pattern of a known error-free reference photomask against which the specimen photomask is compared. The filter is opaque in the areas corresponding to spatial frequency components of the error-free Fourier transform pattern and is transparent in areas not included in the error-free Fourier transform pattern. Neither the Watkins patent nor the Mathisen patent specifies the design parameters of the lens. The Mathisen patent states only that the lens is of suitable numerical aperture and magnification power to cover the area of the specimen photomask.
The spatial frequency components corresponding to the defects in the specimen photomask are largely transmitted through the spatial filter and can be processed in either one of two ways. In the Watkins system, the light transmitted through the spatial filter strikes another double-convex lens that is properly positioned to define an image of the specimen photomask, absent any information blocked by the spatial filter. The imaging light not blocked by the spatial filter appears in locations that represent the position in the specimen photomask where defects are present. In the Mathisen system, the light transmitted through the spatial filter is sensed by a photodetector that produces an output signal which activates a "no-go" alarm.
The Watkins and Mathisen patents imply that systems of the type they describe require both inter- and intra-die interference pattern information to determine the presence of defects in the specimen pattern. The inter-die interference pattern information is of particular concern because it consists of very closely spaced light spots that are extremely difficult to resolve by a Fourier transform lens. The realization of such a lens is further complicated for inspection systems that use an inverse Fourier transform lens to form an image of the specimen pattern from the Fourier transform light pattern. The reason is that the design of each of the lenses is compromised to accomplish an overall system design that accomplishes both the Fourier transform pattern and image forming functions. It is, therefore, exceedingly difficult to obtain from such a system design the resolution required to acquire inter-die interference pattern information. The above lens design problem is encountered in systems of the type that simultaneously inspects the entire area of each of the dies of a specimen photomask array and, as a consequence, renders such systems unreliable and impracticable for commercial use.